Organic electroluminescent element and full color display

ABSTRACT

A organic electroluminescent element is disclosed which comprises a light emission layer containing a fluorescent compound and a phosphorescent compound, the fluorescent compound having a nitrogen atom number to carbon atom number ratio in the molecule (N/C) of from 0 to 0.05, wherein the maximum emission wavelength of light emitted according to electroluminescence of the element is longer than the maximum fluorescence wavelength of the Fluorescent compound.

This is a Divisional Application of parent application U.S. Ser. No.10/167,120, filed on Jun. 10, 2002, now U.S. Pat. No. 6,855,438, andhereby incorporates by reference the entire disclosure of the parent.

FIELD OF THE INVENTION

This invention relates to an organic electroluminescent (hereinafterreferred to also as EL) element, and a display, and particularly to anorganic electroluminescent element excellent in the luminance of emittedlight-and a display comprising the organic electroluminescent element.

BACKGROUND OF THE INVENTION

As an emission type electronic displaying device, there is anelectroluminescence device (ELD). As materials constituting the ELD,there is an inorganic electroluminescent element or an organicelectroluminescent element. The inorganic electroluminescent element hasbeen used for a plane-shaped light source, but a high voltagealternating current has been required to drive the element. An organicelectroluminescent element has a structure in which a light emissionlayer containing a light emission compound is arranged between a cathodeand an anode, and an electron and a positive hole were injected into thelight emission layer and recombined to form an exciton. The elementemits light, utilizing light (fluorescent light or phosphorescent light)generated by deactivation of the exciton, and the element can emit lightby applying a relatively low voltage of from several to several decadevolts. Further, the element has a wide viewing angle and a highvisuality since the element is of self light emission type, and theelement is a complete solid element, and the element is noted from theviewpoint of space saving and portability.

However, in the organic EL element for practical use, an organic ELelement is required which efficiently emits light with high luminance ata lower power.

In U.S. Pat. No. 3,093,796, there is disclosed an element with longlifetime emitting light with high luminance in which stilbenederivatives, distyrylarylene derivatives or tristyrylarylene derivativesare doped with a slight amount of a fluorescent compound.

An element is known which comprises an organic light emission layercontaining an 8-hydroxyquinoline aluminum complex as a host compounddoped with a slight amount of a fluorescent compound (Japanese PatentO.P.I. Publication No. 63-264692), and an element is known whichcomprises an organic light emission layer containing an8-hydroxyquinoline aluminum complex as a host compound doped with aquinacridone type dye (Japanese Patent O.P.I. Publication No. 3-255190).

When light emitted through excited singlet state is used, the upperlimit of the external quantum efficiency (next) is considered to be atmost 5%, as the generation ratio of singlet excited species to tripletexcited species is 1:3, that is, the generation probability of excitedspecies capable of emitting light is 25%, and further, external lightemission efficiency is 20%. Since an organic EL element, employingphosphorescence through the excited triplet, was reported by PrincetonUniversity (M. A. Baldo et al., Nature, 403, 17, p. 151-154 (1998)),study on materials emitting phosphorescence at room temperature has beenactively made. As the upper limit of the internal quantum efficiency ofthe excited triplet is 100%, the light emission efficiency of theexcited triplet is theoretically four times that of the excited singlet.Accordingly, light emission employing the excited triplet exhibits thesame performance as a cold cathode tube, and can be applied toillumination.

It is necessary that when a phosphorescent material is used as a dopant,the maximum wavelength of light which a host compound emits, be in theregion shorter than the maximum wavelength of light which thephosphorescent material emits, but in addition, there exist otherrequisites to be satisfied.

Several proposals with respect to the phosphorescent material were madein “The 10^(th) International Workshop On Inorganic and OrganicElectroluminescence (EL '00, Hamamatsu). For example, Ikai et al. use, ahole transporting compound as a dopant of a phosphorescent material, M.E. Tompson et al. use, as a host compound of a phosphorescent material,various kinds of electron transporting compounds, which are doped with anew iridium complex, and Tsutsui et al. obtain high light emissionefficiency due to introduction of a hole blocking layer.

The host compounds of phosphorescent compounds are disclosed in forexample, C. Adachi et al., “Appl. Phys. Lett., 77, pp. 904 (2000)”, butan approach from a new aspect with respect to characteristics requiredin the host compounds is necessary to obtain an organicelectroluminescent element emitting light with high luminance.

SUMMARY OF THE INVENTION

The present invention has been made in order to improve emissionluminance. An object of the present invention is to provide an organicelectroluminescent element emitting light with high emission luminance,and a display employing the organic electroluminescent element whichemits light with high emission luminance at reduced power consumption.

BRIEF EXPLANATION OF THE DRAWING

FIG. 1 is a schematic drawing of a displaying section of a full colordisplay employing an active matrix system.

DETAILED DESCRIPTION OF THE INVENTION

The object of the invention has been attained by the followingconstitutions:

1. An organic electroluminescent element comprising a light emissionlayer containing a fluorescent compound and a phosphorescent compound,the fluorescent compound having a nitrogen atom number to carbon atomnumber ratio in the molecule (N/C) of from 0 to 0.05, wherein themaximum emission wavelength of light emitted according toelectroluminescence of the element is longer than the maximumfluorescence wavelength of the fluorescent compound.

2. The organic electroluminescent element of item 1, wherein thenitrogen atom number to carbon atom number ratio (N/C) in thefluorescent compound molecule is in the range of from 0 to 0.03.

3. An organic electroluminescent element comprising a light emissionlayer containing a fluorescent compound and a phosphorescent compound,the fluorescent compound having a nitrogen atom number to carbon atomnumber ratio in the molecule (N/C) of from more than 0 to less than0.05, wherein the maximum emission wavelength of light emitted accordingto electroluminescence of the element is longer than the maximumfluorescence wavelength of the fluorescent compound.

4. The organic electroluminescent element of item 3, wherein thefluorescent compound has a nitrogen atom number to carbon atom numberratio in the molecule (N/C) of from more than 0 to 0.03.

5. The organic electroluminescent element of items 1 to 4, wherein themaximum fluorescence wavelength of the fluorescent compound is in therange of from 350 to 440 nm.

6. The organic electroluminescent element of items 1 to 4, wherein themolecular weight of the fluorescent compound is not less than 600.

7. The organic electroluminescent element of items 1 to 4, wherein thephosphorescent compound has a phosphorescent quantum yield of not lessthan 0.01 at 25° C. in its solution.

8. The organic electroluminescent element of items 1 to 4, wherein ahole transporting layer containing at least one fluorescent compound oran electron transporting layer containing at least one fluorescentcompound is provided adjacent to the light emission layer, the maximumfluorescence wavelength of the fluorescent compound being in the rangeof from 350 to 440 nm.

9. The organic electroluminescent element of items 1 to 4, wherein themaximum fluorescence wavelength of the fluorescent compound is in therange of from 390 to 410 nm.

10. The organic electroluminescent element of item 8, wherein themaximum fluorescence wavelength of the fluorescent compound contained inthe light emission layer and the hole transporting layer or the electrontransporting layer is in the range of from 390 to 410 nm.

11. The organic electroluminescent element of items 1 to 4, wherein theorganic electroluminescent element further comprises a cathode, and atleast one cathode buffer layer is provided between the light emissionlayer and the cathode.

12. The organic electroluminescent element of items 1 to 4, wherein thephosphorescent compound is a heavy metal complex compound.

13. The organic electroluminescent element of items 1 to 4, wherein thephosphorescent compound is a metal complex having a metal belonging to agroup VIII. of the periodic table as a center metal.

14. The organic electroluminescent element of items 1 to 4, wherein thephosphorescent compound is an osmium complex, an iridium complex or aplatinum complex.

15. The organic electroluminescent element of items 1 to 4, wherein theelement further comprises a fluorescent compound having a maximumfluorescence wavelength in the region longer than a maximumphosphorescence wavelength of the phosphorescent compound.

16. An organic electroluminescent element comprising a light emissionlayer containing a fluorescent compound and a phosphorescent compound,the maximum emission wavelength of light emitted according toelectroluminescence of the element being longer than the maximumfluorescence wavelength of the fluorescent compound, wherein thefluorescent compound is represented by the following formula (I):

wherein R₁ and R₂ independently represent a substituent; Ar representsan aromatic hydrocarbon ring or an aromatic heterocyclic ring, providedthat each ring may have a substituent; and n is an integer of from 0 to3.

17. An organic electroluminescent element comprising a light emissionlayer containing a fluorescent compound and a phosphorescent compound,the maximum emission wavelength of light emitted according toelectroluminescence of the element being longer than the maximumfluorescence wavelength of the fluorescent compound, wherein thefluorescent compound is represented by the following formula (II):

wherein R₆, R₇ and R₈ independently represent a substituent selectedfrom an alkyl group, a cycloalkyl group, an aryl group, a halogen atom,an alkoxy group, an aryloxy group and a heterocyclic group; and n4, n5and n6 independently represent an integer of from 0 to 7.

18. An organic electroluminescent element comprising a light emissionlayer containing a fluorescent compound and a phosphorescent compound,the maximum emission wavelength of light emitted according toelectroluminescence of the element being longer than the maximumfluorescence wavelength of the fluorescent compound, wherein thefluorescent compound is represented by the following formula (III):

wherein R₁₁ through R₁₆, and X₁ through X₉ independently represent ahydrogen atom or a substituent, and may be the same or different,provided that the following expression is satisfied:

Es_(R11)+Es_(R12)+Es_(R13)+Es_(R14)+Es_(R15)+Es_(R16)≦−2.0 whereinEs_(R11), Es_(R12), Es_(R13), Es_(R14), Es_(R15) and Es_(R16) representthe steric parameter of R₁₁, R₁₂, R₁₃, R₁₁₄, R₁₅, and R₁₆, respectively.

19. An organic electroluminescent element comprising a light emissionlayer containing a fluorescent compound and a phosphorescent compound,the maximum emission wavelength of light emitted according toelectroluminescence of the element being longer than the maximumfluorescence wavelength of the fluorescent compound, wherein thefluorescent compound is represented by the following formula (IV):

wherein R₁₀₁ through R₁₂₈ independently represent a hydrogen atom or asubstituent, provided that at least one of R₁₀₁ through R₁₀₄ representsa substituent.

20. An organic electroluminescent element comprising a light emissionlayer containing a fluorescent compound and a phosphorescent compound,the maximum emission wavelength of light emitted according toelectroluminescence of the element being longer than themaximum-fluorescence wavelength of the fluorescent compound, wherein thefluorescent compound is represented by the following formula (V):

wherein R₂₀₁ through R₂₀₆ independently represent a hydrogen atom or asubstituent.

21. The organic electroluminescent element of items 16 to 20, whereinthe nitrogen atom number to carbon atom number ratio (N/C) in themolecule of the fluorescent compound represented by formula (I), (II),(III), (IV), or (V) is in the range of from 0 to 0.05.

22. The organic electroluminescent element of items 16 to 20, whereinthe nitrogen atom number to carbon atom number ratio (N/C) in themolecule of the fluorescent compound represented by formula (I), (II),(III), (IV), or (V) is in the range of from more than 0 to less than0.05.

23. The organic electroluminescent element of items 16 to 20, whereinthe phosphorescent compound is a heavy metal complex compound.

24. The organic electroluminescent element of items 16 to 20, whereinthe phosphorescent compound is a metal complex compound having a metalbelonging to a group VIII of the periodic table as a center metal.

25. The organic electroluminescent element of items 16 to 20, whereinthe phosphorescent compound is an osmium complex, an iridium complex ora platinum complex.

26. The organic electroluminescent element of items 16 to 20, whereinthe element further comprises a fluorescent compound having a maximumfluorescence wavelength in the region longer than a maximumphosphorescence wavelength of the phosphorescent compound.

27. A display comprising the organic electroluminescent element of anyone of items 1 through 26.

28. A full color display comprising two of the organicelectroluminescent element of any one of items 1 through 26, wherein thetwo elements are arranged on the same substrate, and emit light having amaximum emission wavelength different from each other.

101. An organic electroluminescent element comprising a light emissionlayer containing a fluorescent compound and a phosphorescent compound,the fluorescent compound having a nitrogen atom number to carbon atomnumber ratio (N/C) in the molecule of from 0 to 0.05, wherein themaximum emission wavelength of light emitted according toelectroluminescence of the element is longer than the maximumfluorescence wavelength of the fluorescent compound.

102. The organic electroluminescent element of item 101, wherein themaximum fluorescence wavelength of the fluorescent compound is in therange of from 350 to 440 nm.

103. The organic electroluminescent element of item 101 or 102, whereinthe nitrogen atom number to carbon atom number ratio (N/C) in thefluorescent compound molecule is in the range of from 0 to 0.03.

104. The organic electroluminescent element of item 101 or 102, whereinthe molecular weight of the fluorescent compound is not less than 600.

105. The organic electroluminescent element of any one of items 101through 104, wherein the phosphorescent compound has a phosphorescentquantum yield of not less than 0.01 at 25° C. in its solution.

106. The organic electroluminescent element of any one of items 101through 105, wherein a hole transporting layer containing at least onefluorescent compound or an electron transporting layer containing atleast one fluorescent compound is provided adjacent to the lightemission layer, the maximum fluorescence wavelength of the fluorescentcompound being in the range of from 350 to 440 nm.

107. The organic electroluminescent element of any one of items 101through 105, wherein the maximum fluorescence wavelength of thefluorescent compound is in the range of from 390 to 410 nm.

108. The organic electroluminescent element of item 106, wherein themaximum fluorescence wavelength of the fluorescent compound contained inthe light emission layer and the hole transporting layer or the electrontransporting layer is in the range of from 390 to 410 nm.

109. The organic electroluminescent element of any one of items 101through 108, wherein at least one cathode buffer layer is providedbetween the light emission layer and a cathode.

110. An organic electroluminescent element comprising a light emissionlayer containing a fluorescent compound and a phosphorescent compound,the maximum emission wavelength of light emitted according toelectroluminescence of the element being longer than the maximumfluorescence wavelength of the fluorescent compound, wherein thefluorescent compound is represented by formula (I) as described above.

111. An organic electroluminescent element comprising a light emissionlayer containing a fluorescent compound and a phosphorescent compound,the maximum emission wavelength of light emitted according toelectroluminescence of the element being longer than the maximumfluorescence wavelength of the fluorescent compound, wherein thefluorescent compound is represented by formula (II) as described above.

112. An organic electroluminescent element comprising a light emissionlayer containing a fluorescent compound and a phosphorescent compound,the maximum emission wavelength of light emitted according toelectroluminescence of the element being longer than the maximumfluorescence wavelength of the fluorescent compound, wherein thefluorescent compound is represented by formula (III) as described above.

113. An organic electroluminescent element comprising a light emissionlayer containing a fluorescent compound and a phosphorescent compound,the maximum emission wavelength of light emitted according toelectroluminescence of the element being longer than the maximumfluorescence wavelength of the fluorescent compound, wherein thefluorescent compound is represented by formula (IV) as described above.

114. An organic electroluminescent element comprising a light emissionlayer containing a fluorescent compound and a phosphorescent compound,the maximum emission wavelength of light emitted according toelectroluminescence of the element being longer than the maximumfluorescence wavelength of the fluorescent compound, wherein thefluorescent compound is represented by formula (V) as described above.

115. The organic electroluminescent element of any one of items 110through 114, wherein the nitrogen atom number to carbon atom numberratio (N/C) in the molecule of the fluorescent compound represented byformula (I), (II), (III), (IV), or (V) is in the range of from 0 to0.05.

116. The organic electroluminescent element of any one of items 101through 115, wherein the phosphorescent compound is a heavy metalcomplex compound.

117. The organic electroluminescent element of item 116, wherein thephosphorescent compound is a metal complex compound containing a metalbelonging to a group VIII of the periodic table as a center metal.

118. The organic electroluminescent element of item 116, wherein thephosphorescent compound is an osmium complex, an iridium complex or aplatinum complex.

119. The organic electroluminescent element of any one of items 101through 118, wherein the element further comprises a fluorescentcompound having a maximum fluorescence wavelength in the region longerthan a maximum phosphorescence wavelength of the phosphorescentcompound.

120. A display comprising the organic electroluminescent element of anyone of items 101 through 119.

121. A full color display comprising two or more of the organicelectroluminescent element of items 101 through 119, wherein theelements are arranged on the same substrate, and emit light having amaximum emission wavelength different from each other.

The present invention will be detailed below.

The present inventors have made an extensive study on a fluorescentcompound used as a host compound of a phosphorescent compound, and havefound that there exists some relationship between the luminance ofemitted light of an electroluminescent element and a nitrogen atomnumber to carbon atom number ratio (N/C) in a molecule of the hostcompound contained in the electroluminescent element. As a result, ithas been proved that a host compound having a relatively low N/Cincreases the emitted light luminance. It is considered that the emittedlight luminance is restricted by some action of the nitrogen atom whichis contained in the molecule of a host compound having a relatively highN/C. Accordingly, in order to increase emitted light luminance of anorganic electroluminescent element employing a phosphorescent compoundas a dopant, it is effective to reduce an N/C of the phosphorescentcompound.

In the invention, the fluorescent compound is a compound in which lightis emitted through light excited singlet state in which two electronspins are in reversely parallel with each other, and the phosphorescentcompound is a compound in which light is emitted through light excitedtriplet state in which two electron spins are in parallel with eachother. Herein, the phosphorescent compound in the invention isconsidered to form excited triplet state at room temperature (from 15 to30° C.) through energy transfer from the exited singlet state or excitedtriplet state of the fluorescent compound described above.Phosphorescent compounds have been considered to be capable of emittingphosphoresce only at a low temperature such as 77° K. However, since inrecent years, compounds capable of emitting phosphoresce at roomtemperature have been found, many compounds, mainly heavymetal-containing complexes such as iridium complexes, have beensynthesized and studied (see for example, S. Lamansky et al, J. Am.Chem. Soc., 123, pp. 4304, 2001).

In the invention, the maximum fluorescence wavelength of the fluorescentcompound as a host compound is a wavelength giving the maximumfluorescent intensity in the fluorescent spectra of a 100 nm thick layerof the fluorescent compound deposited on a glass plate.

In the invention, it has been confirmed that the organicelectroluminescent element comprising a light emission layer containinga phosphorescent compound and a fluorescent compound having a nitrogenatom number/carbon atom number ratio (N/C) in the molecule of from 0 to0.05 emits light with high luminance. Accordingly, in the invention, thefluorescent compound as a host compound, which is used in combinationwith a phosphorescent compound, is preferably a compound having in itsmolecule an N/C of from 0 to 0.05. Although the reason is not completelyelucidated, as described above, it is considered that luminance ofemitted light is restricted by some action of the nitrogen atom which iscontained in the molecule of a host compound having a relatively highN/C.

Further, in the invention, a fluorescent compound as a host compoundhaving in the molecule an N/C of from 0 to 0.03 is preferably used incombination with a phosphorescent compound in that lifetime of emittedlight is longer. Although the reason is not completely elucidated, it isconsidered that in order to obtain relatively long lifetime of emittedlight, the use of a host compound having a nitrogen atom is preferablebut the use of a host compound having an N/C not less than a certainvalue restricts the lifetime due to some action of the nitrogen atom.

In the invention, the phosphorescent maximum wavelength of thephosphorescent compound as a dopant contained in the lightemission-layer is longer than the maximum fluorescence wavelength of thefluorescent compound as a host compound, whereby an organicelectroluminescence (EL) element can be obtained which employs lightemission due to excited triplet of the phosphorescent compound as adopant. Accordingly, the organic electroluminescent element of theinvention emits light with an electroluminescent maximum wavelengthlonger than the maximum fluorescence wavelength of the fluorescentcompound (a wavelength giving the maximum fluorescent intensity which isdetermined from the fluorescent spectra of a layer of the fluorescentcompound which is deposited on a glass plate to give a thickness of 100nm).

In the invention, the maximum fluorescence wavelength of the fluorescentcompound used as a host compound is in the range of preferably from 350to 440 nm, and more preferably from 390 to 410 nm.

A lower organic compound is poor in thermal stability due to its lowmolecular weight, and may not emit light with sufficiently highluminance. In the invention, the molecular weight of the fluorescentcompound as a host compound of the phosphorescent compound is preferablynot less than 600 in view of thermal stability.

In the invention, the phosphorescent quantum yield of the phosphorescentcompound at 25° C. in its solution is preferably not less than 0.001,more preferably not less than 0.01, and most preferably not less than0.1.

Next, the measurement means and theory of the quantum yield φ_(p) ofexcited triplet state will be explained.

Energy transition from excited singlet state to the ground state is madeat a rate constant of nonradiative transition k_(sn) and at a rateconstant k_(f) of fluorescent radiation k_(f), whereby excited energy islost. In addition to this, energy transition from excited singlet stateto the excited triplet state is made at a rate constant K_(isc). Herein,lifetime of the excited singlet state τ_(s) is defined by the followingformula:τ_(s)=(k_(sn) +k _(f) +k _(isc))⁻¹

Fluorescent quantum yield φ_(f) is represented by the following formula:φ_(f) =k _(f)·τ_(s)

Energy transition from excited triplet state to the ground state is madeat a rate constant of nonradiative transition k_(tn) and at a rateconstant of phosphorescent radiation k_(p), whereby excited energy islost. Herein, lifetime of the excited triplet state τ_(t) is defined bythe following formula:τ_(t)=(k _(tn) +k _(p))⁻¹

τ_(t) is in the range of ordinarily 10⁻⁶ to 10⁻³ seconds, but may be upto several seconds.

Employing quantum yield of excited triplet state generation φ_(ST),phosphorescent quantum yield φ_(p) is represented as follows:φ_(p)=φ_(ST) ·k _(p)τ_(t)

The parameters described above can be measured according to a methoddescribed in the fourth edition, Jikken Kagaku Koza 7, Bunko II, p. 398(1992) (published by Maruzen).

In the invention, the steric parameter Es of the substituent is asubstitution constant defined by Taft and described in, for example,“Relation between Structure and Activity of Medicine” extra issue No.122, Konando. In the invention, Es value is a value based on a hydrogenatom, namely the value of Es(H=0), and calculated by subtracting 1.24from the value of Es(CH₃) based on a methyl group Typical examples ofthe value are shown in Table 1.

TABLE 1 Substituent Es value H 0 CH₃ −1.24 C₂H₅ −1.31 i-C₃H₇ −1.71t-C₄H₉ −2.78 F −0.46 Cl −0.97 Br −1.16 CF₃ −2.40 CCl₃ −3.30 OCH₃ −0.55OH −0.55 SH −1.07 CN −0.51

The light emission layer will be detailed below.

The light emission layer herein refers to a layer emitting light whenelectric current is supplied to an electrode comprised of a cathode andan anode. Typically, the light emission layer is a fluorescentcompound-containing layer emitting light when electric current issupplied to an electrode comprised of a cathode and an anode. Theorganic electroluminescent element (EL element) basically has astructure in which a light emission layer is put between a pair ofelectrodes. The organic EL element of the invention has a structure inwhich in addition to the light emission layer, a hole transportinglayer, an electron transporting layer, an anode buffer layer or acathode buffer layer is optionally provided between a cathode and ananode.

In concrete, the following structures are included.

-   -   (i) Anode/Light emission layer/Cathode    -   (ii) Anode/Hole transporting layer/Light emission layer/Cathode    -   (iii) Anode/Light emission layer/Electron transporting        layer/Cathode    -   (iv) Anode/Hole transporting layer/Light emission layer/Electron        transporting layer/Cathode    -   (v) Anode/Anode buffer layer/Hole transporting layer/Light        emission layer/Electron transporting layer/Cathode buffer        layer/Cathode

As a method of forming a light emission layer employing the compoundsmentioned above, there is a known method for forming a thin film such asa deposition method, a spin-coat method, a casting method and an LBmethod. The light emission layer is preferably a molecular depositlayer. The molecular deposit layer herein refers to a layer formed bydeposition of the above compounds in a gaseous state, or to a layerformed by solidification of the compounds in a melted state or aliquefied state. The molecular deposit layer is distinguished from athin layer (molecular cumulation layer) formed by an LB method by astructural difference, for example, an aggregated structure or a higherorder structure, or a functional difference resulting from thestructural difference.

Moreover, the light emission layer can be formed by the method such asthat described in Japanese Patent O.P.I. Publication No. 57-51781, bywhich the above compound as a light emission material is dissolved in asolvent together with a binder such as a resin, and the thus obtainedsolution is formed into a thin layer by a method such as a spin-coatmethod. Thickness of the emission layer thus formed is not speciallyrestricted. Although the thickness of the layer thus formed isoptionally selected, the thickness is preferably within the range offrom 5 nm to 5 μm.

The phosphorescent compound in the invention is concretely a heavy metalcomplex compound, preferably a metal complex compound containing a metalbelonging to a group VIII of the periodic table, and more preferably ametal complex compound containing osmium, iridium or platinum.

The phosphorescent compound has a phosphorescent quantum yield in thesolution at 25° C. of preferably not less than 0.001 at 25° C., asdescribed above, and has a maximum phosphorescence wavelength longerthan a maximum fluorescence wavelength of a fluorescent compound as ahost compound. Employing the phosphorescent compound emitting aphosphorescent light with a maximum phosphorescence wavelength longerthan a maximum fluorescence wavelength of the fluorescent compound as ahost compound, that is, employing phosphorescence emitted by thephosphorescent compound or a triplet state, an EL element can beobtained which emits electroluminescence light having wavelengths longerthan a maximum fluorescence wavelength of the fluorescent compound. Themaximum phosphorescence wavelength of the phosphorescent compound usedis not specifically limited, but the wavelength of emitted light can betheoretically varied selecting a center metal, a ligand or a substituentof the ligand contained in the phosphorescent compound.

For example, employing a fluorescent compound having a maximumfluorescent wavelength in the range of from 35O to 440 nm as a hostcompound, and an iridium complex compound having a phosphorescent lightin a green region, an EL element can be obtained which emitselectroluminescence light in the green region.

Examples of the phosphorescent compound used in the invention will belisted below, but are not limited thereto. These compounds can besynthesized according to a method described in for example, Inorg. Chem.40, 1704-1711.

As another embodiment of the invention, there is an organicelectroluminescent element comprising, in addition to a fluorescentcompound (A) as a host compound and a phosphorescent compound, afluorescent compound (B) having a phosphorescent maximum wavelength inthe wavelength region longer than the maximum wavelength of lightemitted by the phosphorescent compound. In this case, theelectro-luminescence of the EL element is emitted from the fluorescentcompound (B) to which energy is transferred from the fluorescentcompound (A) and the phosphorescent compound. The fluorescent compound(B) preferably has a high quantum yield in the for m of solution.Herein, the quantum yield is preferably not less than 10%, and morepreferably not less than 30%. Examples of the fluorescent compound (B)include a qumarine dye, a cyanine dye, a chloconium dye, a squaleniumdye, an oxobenzanthracene dye, a fluorescene dye, a rhodamine dye, apyrylium dye, a perylene dye, a stilbene dye, a polythiophene dye, and afluorescent compound of a rare earth element complex type.

The fluorescent quantum yield herein can be measured according to amethod described in the fourth edition, Jikken Kagaku Koza 7, Bunko II,p. 362 (1992) (published by Maruzen). In the invention, tetrahydrofuranis employed as a solvent for measurement.

In the invention, it is preferred that a fluorescent compound, having anitrogen atom number-to-carbon atom number ratio (N/C) in the moleculeof from 0 to 0.05, is used as a host compound in combination with aphosphorescent compound. This can provide an organic EL element having ahigh luminance and a long emission lifetime, but from another viewpoint,it is useful in the invention to employ the compounds represented byformulae (I) through (V) as a host compound which is used in combinationwith the phosphorescent compound.

The compounds represented by formulae (I) through (V) in the inventionwill be explained below.

In formula (I), n is an integer of from 0 to 3, and R₁ and R₂independently represent a substituent. The substituent is preferably analkyl group such as a methyl group, an ethyl group, an i-propyl group, ahydroxyethyl group, a methoxymethyl group, a trifluoromethyl group and at-butyl group, a halogen atom such as a fluorine atom and a chlorineatom, or an alkoxy group such as a methoxy group, an ethoxy group, ani-propoxy group and a butoxy group. Ar represents an aromatichydrocarbon ring or an aromatic heterocyclic ring, provided that eachmay have a substituent, and preferably represents a naphthyl group, abinaphthyl group, a quinolyl group, an iso-quinolyl group, abenzoxazolyl group and a benzimidazolyl group, when n are an integer of2 or more, plural R₁s and R₂s each may be the same or different.

In Formula (II), n4, n5 and n6 independently represent an integer offrom 0 to 7. R₆, R₇ and R₈ independently represent a substituentselected from an alkyl group, a cycloalkyl group, an aryl group, ahalogen atom, an alkoxy group, an aryloxy group and a heterocyclicgroup; and a methyl group and a naphthyl group are particularlypreferred.

When n4, n5 and n6 each are an integer of 2 or more, plural R₆s, R₇s,and R₈s each may be the same or different.

In Formula (III), R₁₁ through R₁₆, and X₁ through X₉ independentlyrepresent a hydrogen atom or a substituent, and may be the same ordifferent, provided that the sum of Es_(R11) through Es_(R16), whichrepresent the steric parameter of R₁₁ through R₁₆, respectively,satisfies the following expression:Es_(R11)+Es_(R12)+Es_(R13)+Es_(R14)+Es_(R15)+Es_(R16)≦−2.0;

The adjacent substituents may be condensed with each other to form aring structure. The substituents represented by R₁₁ through R₁₆ arepreferably a methyl group, an ethyl group, an isopropyl group, a t-butylgroup or a trifluoromethyl group. The substituents represented by X₁through X₉ are preferably an alkyl group, an aryl group, a heterocyclicgroup, a halogen atom, an alkoxy group and an amino group. X₂, X₅ and X₈each are preferably an aryl group or an amino group, and especiallypreferably a diarylamino group.

In formula (IV), R₁₀₁ through R₁₂₈ independently represent a hydrogenatom or a substituent, provided that at least one of R₁₀₁ through R₁₀₄represents a substituent. When R₁₀₁ through R₁₂₈ represent asubstituent, the substituent is preferably an alkyl group (for example,a methyl group, an ethyl group, an i-propyl group, a hydroxyethyl group,a methoxymethyl group, a trifluoromethyl group, a perfluoropropyl group,a perfluoro-n-butyl group, a perfluoro-t-butyl group, or a t-butylgroup); a cycloalkyl group (for example, a cyclopentyl group and acyclohexyl group); an aralkyl group (for example, a benzyl group and a2-phenethyl group); an aryl group (for example, as a phenyl group, anaphthyl group, a p-tolyl group and a p-chlorophenyl group); an alkoxygroup (for example, a methoxy group, an ethoxy group, an i-propoxy groupand a butoxy group); an aryloxy group (for example, a phenoxy group) oran arylamino group (for example, a diphenylamino group). The foregoinggroups further may have a substituent, examples of which include ahydrogen atom, a halogen atom, a trifluoromethyl group, an alkyl group,an aryl group, an alkoxy group, an aryloxy group, an alkylthio group, adialkylamino group, a dibenzylamino group, and a diarylamino group.

In formula (IV), the substituent represented by R₁₀₁ through R₁₀₄ ispreferably an alkyl group, and two or four substituents of R₁₀₁ throughR₁₀₄ are more preferably methyl groups.

In formula (V), R₂₀₁, R₂₀₂, R₂₀₃, R₂₀₄, R₂₀₅, and R₂₀₆ independentlyrepresent a hydrogen atom, or a substituent. When R₂₀₁ through R₂₀₆independently represent a substituent, the substituent is preferablythose denoted in R₁₀₁ through R₁₂₈ above, more preferably a substitutedor unsubstituted aryl group, and most preferably substituted orunsubstituted phenyl group.

Of the compounds represented by formulae (I) through (V), compoundshaving a nitrogen atom number-to-carbon atom number ratio (N/C) in themolecule of not more than 0.05 are preferable, and compounds having anitrogen atom number-to-carbon atom number ratio (N/C) in the moleculeof not more than 0.03 are more preferable.

Examples of compounds represented by formulae (I) through (V) will belisted below, but are not limited thereto.

Besides the compounds represented by formulae (I) through (V) above,compounds having a nitrogen atom number-to-carbon atom number ratio(N/C) in the molecule of from 0 to 0.05, which can be used in theinvention, include the following exemplified compounds.

Color of light emitted from the fluorescent compound according to theinvention is measured by a spectral light meter CS-1000, manufactured byMinolta Co., Ltd., and expressed according to CIE chromaticity diagramdescribed in FIG. 4.16 on page 108 of “Shinpen Shikisai Kagaku Handbook”(Coloring Science Handbook, New Edition), edited by Nihon ShikisaiGakkai, published by Todai Shuppan Kai.

Compounds represented by formulae (I) through (V) have a high glasstransition point (Tg), and therefore, provide high thermal stability asmaterials for an organic electroluminescence element. The Tg, which thecompounds have, is preferably not less than 100° C.

The molecular weight of the compounds represented by formulae (I)through (V) is in the range of preferably from 600 to 5000. Thecompounds having the molecular weight of such a range can provide alight emission layer capable of being easily formed by an vacuumdeposition method, and therefore, an organic EL element can be easilymanufactured. Further, thermal stability of the fluorescent compound inthe organic EL element is higher.

Layers such as a hole injecting layer, a hole transporting layer, anelectron injecting layer and an electron transporting layer other thanthe light emission layer, which constitute the organic EL element, willbe explained below.

In the invention, a hole injecting layer or a hole transporting layerhas a function of transporting the positive hole injected from the anodeto the light emission layer. Many positive holes can be injected in alower electric field by the presence of the hole injecting layer or thehole transporting layer between the anode and the light emission layer.Moreover, an element can be obtained which increases a light emissionefficiency and has an excellent light emission ability, since theelectrons injected into the light emission layer from the cathode, theelectron injecting layer or the electron transporting layer areaccumulated at the interface in the light emission layer by a barrier toelectrons existing at the interface between the light emission layer andthe hole injecting layer or the hole transporting layer.

The material for the hole injecting layer and the hole transportinglayer (hereinafter referred to also as a hole injecting material and ahole transporting material, respectively) can be optionally selectedfrom known materials without any limitation as far as they have afunction capable of transporting the positive hole injected from theanode to the light emission layer. Such materials include those employedfor hole transporting materials in conventional photoconductive elementsor known materials used in the hole injecting layer or hole transportinglayer of conventional EL elements.

The hole injecting material or the hole transporting material describedabove may be either an organic substance or an inorganic substance aslong as it has a hole injecting ability, a hole transporting ability oran ability to form a barrier to electron. Examples of the hole injectingmaterial or the hole transporting material include a triazolederivative, an oxadiazole derivative, an imidazole derivative, apolyarylalkane derivative, a pyrazoline derivative and a pyrazolonederivative, a phenylenediamine derivative, an arylamine derivative, anamino substituted chalcone derivative, an oxazole derivative, a styrylanthracene derivative, a fluorenone derivative, a hydrazone derivative,a stilbene derivative, a silazane derivative, an aniline copolymer, andan electroconductive oligomer particularly a thiophene oligomer. As thehole injecting material or the hole transporting material, thosedescribed above are used, but a porphyrin compound, an aromatic tertiaryamine compound, or a styrylamine compound is preferably used, and anaromatic tertiary amine compound is more preferably used.

Typical examples of the aromatic tertiary amine compound and styrylaminecompound include N,N,N′,N′-tetraphenyl-4,4′-diaminophenyl,N,N′-diphenyl-N,N′-bis(3-methylphenyl)-[1,1′-biphenyl]-4,4′-diamine(TPD), 2,2′-bis(4-di-p-tolylaminophenyl)propane,1,1′-bis(4-di-p-tolylaminophenyl)cyclohexane,N,N,N′,N′-tetra-p-tolyl-4,4′-diaminobiphenyl,1,1′-bis(4-di-p-tolylaminophenyl)-4-phenylcyclohexane,bis(4-dimethylamino-2-methylphenyl)-phenylmethane,bis(4-di-p-tolylaminophenyl)-phenylmethane,N,N′-diphenyl-N,N′-di(4-methoxyphenyl)-4,4′-diaminobiphenyl,N,N,N′,N′-tetraphenyl-4,4′-diaminodiphenylether,4,4′-bis(diphenylamino)quardriphenyl, N,N,N-tri(p-tolyl)amine,4-(di-p-tolylamino)-4′-[4-(di-p-tolylamino)styryl]stilbene,4-N,N-diphenylamino-(2-diphenylvinyl)benzene,3-methoxy-4′-N,N-diphenylaminostyrylbenzene, N-phenylcarbazole,compounds described in U.S. Pat. No. 5,061,569 which have two condensedaromatic rings in the molecule thereof such as4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (NPD), and compoundsdescribed in Japanese Patent O.P.I. Publication No. 4-308688 such as4,4′,4″-tris[N-(3-methylphenyl)-N-phenylamino]-triphenylamine (MTDATA)in which three triphenylamine units are bonded in a starburst form.

A polymer in which the material mentioned above is introduced in thepolymer chain or a polymer having the material as the polymer main chaincan be also used.

As the hole injecting material or the hole transporting material,inorganic compounds such as p-Si and p-SiC are usable. The holeinjecting layer or hole transporting layer can be formed by layering thehole injecting material or the hole transporting material by a knownmethod such as a vacuum deposition method, a spin coat method a castingmethod and an LB method. The thickness of the hole injecting layer orthe hole transporting layer is not specifically limited, but isordinarily from 5 nm to 5 μm. The hole injecting layer or the holetransporting layer may be composed of a single layer structurecomprising one or two or more of the materials mentioned above, or ofplural layers comprising the same composition or different composition.

An electron transporting layer which is provided according to necessityis a layer having a function of transporting electrons injected to thecathode to the light emission layer. The material for the electrontransporting layer may be optionally selected from known compounds.

Examples of the material used in the electron transporting layer(hereinafter referred to also as electron transporting material) includea nitro-substituted fluorene derivative, a diphenylquinone derivative, athiopyran dioxide derivative, a heterocyclic tetracarboxylic acidanhydride such as naphthaleneperylene, a carbodiimide, afluorenylidenemethane derivative, an anthraquinodimethane an anthronederivative, and an oxadiazole derivative. Moreover, a thiadiazolederivative which is formed by substituting the oxygen atom in theoxadiazole ring of the foregoing oxadiazole derivative with a sulfuratom, and a quinoxaline derivative having a quinoxaline ring known as anelectron withdrawing group are usable as the electron transportingmaterial.

A polymer in which the material mentioned above is introduced in thepolymer chain or a polymer having the material as the polymer main chaincan be also used.

A metal complex of an 8-quinolinol derivative such as aluminumtris-(8-quinolinol) (Alq₃), aluminum tris-(5,7-dichloro-8-quinolinol),aluminum tris-(5,7-dibromo-8-quinolinol), aluminumtris-(2-methyl-8-quinolinol), aluminum tris-(5-methyl-8-quinolinol), orzinc bis-(8-quinolinol) (Znq), and a metal complex formed by replacingthe center metal of the foregoing complexes with another metal atom suchas In, Mg, Cu, Ca, Sn, Ga or Pb, can be used as the electrontransporting material. Furthermore, a metal free or metal-containingphthalocyanine, and a derivative thereof, in which the molecularterminal is replaced by a substituent such as an alkyl group or asulfonic acid group, are also preferably used as the electrontransporting material. The distyrylpyrazine derivative exemplified as amaterial for an light emission layer may preferably be employed as theelectron transporting material. An inorganic semiconductor such as n-Siand n-SiC may also be used as the electron transporting material in asimilar way as in the hole injecting layer or in the hole transportinglayer.

The electron transporting layer can be formed by layering the compoundsdescribed above by a known method such as a vacuum deposition method, aspin coat method, a casting method and an LB method. The thickness ofthe electron transporting layer is not specifically limited, but isordinarily from 5 nm to 5 μm. The electron transporting layer may becomposed of a single layer structure comprising one or two or more ofthe materials mentioned above, or of plural layers comprising the samecomposition or different composition.

In the invention, the light emission layer contains the fluorescentcompound, but a positive hole or electron transporting layer adjacent tothe light emission layer may contain one or more kinds of fluorescentcompounds having a maximum fluorescence wavelength in the same regionsas the fluorescent compound as a host compound of the phosphorescentcompound described above, whereby an emission efficiency of the ELelement can be increased. The fluorescent compound, which may becontained in the hole or electron transporting layer, is a fluorescentcompound having the maximum fluorescence wavelength in the region offrom 350 to 440 nm, and preferably 390 to 410 nm, as the fluorescentcompound contained in the light emission layer.

A substrate preferably employed for the organic electroluminescentelement of the invention is not restricted to specific kinds ofmaterials such as glass and plastic, as far as it is transparent.Examples of the substrate preferably employed used in the organicelectroluminescent element of the invention include glass, quartz andlight transmissible plastic film.

Examples of the light transmissible plastic film include films such aspolyethyleneterephthalate (PET), polyethylenenaphthalate (PEN),polyethersulfone (PES), polyetherimide, polyetheretherketone,polyphenylenesulfide, polyarylate, polycarbonate (PC), cellulosetriacetate (TAC), cellulose acetate propionate (CAP) and so on.

Preferable examples in the preparation of the organic EL element will bedescribed below.

For one example, the preparation of the EL element having the foregoingconstitution, Anode/Hole injecting layer/Hole transporting layer/Lightemission layer/Electron transporting layer/Electron injectinglayer/Cathode, will be described.

A thin layer of a desired material-for electrode such as a material foran anode is formed on a suitable substrate by a deposition or spatteringmethod, so that the thickness of the layer is not more than 1 μm, andpreferably within the range of from 10 to 200 nm to prepare the anode.Then the hole injecting layer, the hole transporting layer, the lightemission layer, the electron transporting layer and the electroninjecting layer, which constitute the element, are formed on theresulting anode in that order.

A buffer layer (an electrode interface layer) may be provided betweenthe anode and the light emission layer or the hole injecting layer, orbetween the cathode and the light emission layer or the electroninjecting layer.

The buffer layer is a layer provided between the electrode and anorganic layer in order to reduce the driving voltage or to improve oflight emission efficiency. As the buffer layer there are an anode bufferlayer and a cathode buffer layer, which are described in “ElectrodeMaterial” page 123, Div. 2 Chapter 2 of “Organic EL element and itsfrontier of industrialization” (published by NTS Corporation, Nov. 30,1998) in detail.

The anode buffer layer is described in Japanese Patent O.P.I.Publication Nos. 9-45479, 9-260062, and 8-288069 etc., and its examplesinclude a phthalocyanine buffer layer represented by a copperphthalocyanine layer, an oxide buffer layer represented by a vanadiumoxide layer, an amorphous carbon buffer layer, a polymer buffer layeremploying an electroconductive polymer such as polyaniline (emeraldine),and polythiophene, etc.

The cathode buffer layer is described in Japanese Patent O.P.I.Publication Nos. 6-325871, 9-17574, and 9-74586, etc. in detail, and itsexamples include a metal buffer layer represented by a strontium oraluminum layer, an alkali metal compound buffer layer represented by alithium fluoride layer, an alkali earth metal compound buffer layerrepresented by a magnesium fluoride layer, and an oxide buffer layerrepresented by an aluminum oxide or lithium oxide layer.

The buffer layer is preferably very thin and has a thickness ofpreferably from 0.1 to 100 nm depending on kinds of the material used.

A layer having another function may be provided if necessary in additionto the fundamental configuration layers as described above, for examplea hole blocking layer may be added as described in Japanese PatentO.P.I. Publication Nos. 11-204258, and 11-204359, and on page 237 of“Organic EL element and its frontier of industrialization” (published byNTS Corporation, Nov. 30, 1998).

At least one of the cathode buffer layer and anode buffer layer maycontain the compound in the invention, and may function as a lightemission layer.

Electrodes of the organic EL element will be explained below. Theelectrodes consist of a cathode and an anode.

For the anode of the organic EL element, a metal, an alloy, or anelectroconductive compound each having a high working function (not lessthan 4 eV), and mixture thereof are preferably used as the electrodematerial. Concrete examples of such an electrode material include ametal such as Au, and a transparent electroconductive material such asCuI, indium tin oxide (ITO), SnO₂, or ZnO.

The anode may be prepared by forming a thin layer of the electrodematerial according to a depositing or spattering method, and by formingthe layer into a desired pattern according to a photolithographicmethod. When required precision of the pattern is not so high (not lessthan 100 μm), the pattern may be formed by depositing or spattering ofthe electrode material through a mask having a desired form. When lightis emitted through the anode, the transmittance of the anode ispreferably 10% or more, and the sheet resistivity of the anode ispreferably not more than several hundred Ω/□. The thickness of the layeris ordinarily within the range of from 10 nm to 1 μm, and preferablyfrom 10 to 200 nm, although it may vary due to kinds of materials used.

On the other hand, for the cathode, a metal (also referred to as anelectron injecting metal), an alloy, and an electroconductive compoundeach having a low working function (not more than 4 eV), and a mixturethereof is used as the electrode material. Concrete examples of such anelectrode material include sodium, sodium-potassium alloy, magnesium,lithium, a magnesium/copper mixture, a magnesium/silver mixture, amagnesium/aluminum mixture, magnesium/indium mixture, analuminum/aluminum oxide (Al₂O₃) mixture, indium, a lithium/aluminummixture, and a rare-earth metal. Among them, a mixture of an electroninjecting metal and a metal higher in the working function than that ofthe electron injecting metal, such as the magnesium/silver mixture,magnesium/aluminum mixture, magnesium/indium mixture, aluminum/aluminumoxide (Al₂O₃) mixture or lithium/aluminum mixture, is suitable from theview point of the electron injecting ability and resistance tooxidation. The cathode can be prepared forming a thin layer of such anelectrode material by a method such as a deposition or spatteringmethod. The sheet resistivity as the cathode is preferably not more thanseveral hundred Ω/□, and the thickness of the layer is ordinarily from10 nm to 1 μm, and preferably from 50 to 200 nm. It is preferable inincreasing the light emission efficiency that either the anode or thecathode of the organic EL element is transparent or semi-transparent.

A preparation method of the organic EL element will be explained below.

For formation of the thin layer, a vacuum deposition method ispreferably used even though a spin coating method, a casting method anda deposition method can be used. The vacuum deposition method ispreferable since a uniform layer can be formed and a pinhole is formedwith difficulty. Although conditions of the vacuum deposition aredifferent due to kinds of materials used, or an intended crystalline orassociation structure of the molecular deposited layer, the vacuumdeposition is preferably carried out at a boat temperature of from 50°C. to 450° C., at a vacuum degree of from 10⁻⁶ to 10⁻³ Pa, at adeposition speed of from 0.01 to 50 nm/second, and at a substratetemperature of from −50 to 300° C., to form a layer thickness of from 5nm to 5 μm.

As described above, on a suitable substrate, a thin layer of a desiredelectrode material such as an anode material is formed by a depositionor spattering method so that the thickness of the layer is not more than1 μm, preferably within the range of from 10 to 200 nm to prepare theanode. Then the hole injecting layer, the hole transporting layer, thelight emission layer and the electron transporting layer and theelectron injecting layer are formed on the anode as described above.After formation of these layers, a thin layer comprising a material forcathode is formed thereon by, for example, a deposition method orspattering method so that the thickness is not more than 1 μm, andpreferably from 50 to 200 nm, to provide the cathode. Thus a desired ELelement is obtained. It is preferred that the layers from the holeinjecting layer to the cathode are continuously formed under one time ofvacuuming to prepare the organic EL element. Further, the organic ELelement can be prepared in the reverse order, in which the cathode, theelectron injecting layer, the light emission layer, hole injectinglayer, and the anode are formed in that order. Light emission can beobserved when a direct current with a voltage of from about 5 to 40 V isapplied to the thus prepared organic EL element so that the polarity ofthe anode is positive and that of the cathode is negative. When thevoltage is applied in the reverse polarity, no current is generated andlight is not emitted at all. When an alternating voltage is applied,light is emitted only when the polarity of the anode is positive andthat of the cathode is negative. The shape of the wave of thealternating current may be optionally selected.

EXAMPLES

The present invention will be explained in the following examples, butis not limited thereto.

Example 1

Electroluminescence element sample Nos. 1-1 through 1-22 were preparedaccording to the following procedures:

<Preparation of Organic EL Element Sample>

A pattern was formed on a substrate (manufactured by NH Technoglass Co.,Ltd.) composed of a glass plate (100 mm×1000 mm×1.1 mm) and a 150 nm ITO(indium tin oxide) layer as an anode. Then the resulting transparentsubstrate having the ITO transparent electrode was subjected toultrasonic washing in i-propyl alcohol and dried by a dry nitrogen gasand subjected to UV-ozone cleaning for 5 minutes.

Thus obtained transparent substrate was fixed on a substrate holder of avacuum deposition apparatus available in the market. Further, 200 mg ofα-NPD were put in a first resistive heating molybdenum boat, 200 mg ofCBP were put in a second resistive heating molybdenum boat, 200 mg ofbathocuproine (BC) were put in a third resistive heating molybdenumboat, 200 mg of Ir-1 (phosphorescent compound) were put in a fourthresistive heating molybdenum boat, and 200 mg of Alq₃ were put in afifth resistive heating molybdenum boat, and the resulting boats wereset in the vacuum deposition apparatus. Then the pressure in the vacuumtank was reduced to 4×10⁻⁴ Pa, and the boat carrying α-NPD was heated to220° C. by supplying an electric current to the boat so as to form ahole transporting layer having a thickness of 45 nm by depositing α-NPDonto the transparent substrate at a depositing speed of 0.1 nm/sec.Moreover, the boat carrying CBP and the boat carrying Ir-1 were heatedto 220° C. by supplying an electric current to both boats so as toco-deposit the CBP at a depositing speed of 0.01 nm/sec and the Ir-1 ata depositing speed of 0.1 nm/sec onto the hole transporting layer toform a light emission layer of 20 nm. The temperature of the substrateat the time of the deposition was room temperature. Then the boatcarrying BC was heated to 250° C. by supplying an electric current tothe boat so as to deposit the BC onto the light emission layer at adepositing speed of 0.1 nm/sec to form an electron transporting layer of10 nm which could function as a hole blocking layer. Further, the boatcarrying Alq₃ was heated to 250° C. by supplying an electric current tothe boat so as to deposit the Alq₃ onto the electron transporting layerat a depositing speed of 0.1 nm/sec to form an electron transportinglayer of 40 nm. The temperature of the substrate at the time of thedeposition was room temperature.

Next, the vacuum tank was opened, and a stainless steel mask having arectangular hole was set on the electron transporting layer. Further, 3g of magnesium was put in a molybdenum resistive heating boat and 0.5 gof silver was put in a tungsten basket for deposition. The boat and thebasket were set in the vacuum tank. The pressure in the vacuum tank wasreduced to 2×10⁻⁴ Pa. Then the boat carrying magnesium was heated bysupplying an electric current so as to deposit magnesium at a depositionspeed of from 1.5 to 2.0 nm/sec. At this time, the basket carryingsilver was simultaneously heated so as to deposit silver at a depositionspeed of 0.1 nm/sec to form a counter electrode composed of a mixture ofmagnesium and silver. Thus a comparative organic EL element sample No.1-1 was prepared.

Organic EL element sample Nos. 1-2 through 1-22 were prepared in thesame manner as comparative organic EL element sample No. 1-1, exceptthat CBP and the phosphorescent compound in the light emission layerwere replaced with those as shown in Table 2.

The chemical structures of the compounds used above are shown below.

<Evaluation of Luminance of Light Emitted from Organic EL Element SampleNos. 1-1 Through 1-22>

When an initial driving voltage of 3V was applied to organic EL elementsample No. 1-1, an electric current begins flowing and a light wasemitted from the phosphorescent compound which was a dopant in the lightemission layer. When a direct current voltage of 9V was applied toorganic EL element sample No. 1-1 at 23° C. in an atmosphere of a drynitrogen gas, luminance of light emitted from the sample 1-1 wasmeasured according to CS-1000 produced Minolta Co., Ltd. The luminanceof light emitted from the organic EL element sample Nos. 1-2 through1-22 was expressed by a relative value when the luminance of lightemitted from the organic EL element sample No. 1-1 was set at 100. Theresults are shown in Table 2.

TABLE 2 Compound Maximum contained in flourescence Phospho- LuminanceSample light emission wavelength Molecular rescent of emitted EmissionNo. layer N/C (nm) weight compound light color Remarks 1-1  CBP 0.056405 484 Ir-1 100 Green Comp. 1-2  TAZ 0.125 415 347 Ir-1 138 Green Comp.1-3  OXD7 0.2 375 478 Ir-1 135 Green Comp. 1-4  BC 0.077 398 360 Ir-1103 Green Comp. 1-5  TPA 0.076 425 906 Ir-1 86 Green Comp. 1-6  (1)0.0238 385 558 Ir-1 163 Green Inv. 1-7  (6) 0.0152 407 858 Ir-1 180Green Inv. 1-8  (7) 0.0152 405 858 Ir-1 180 Green Inv. 1-9  (9) 0.0208380 628 Ir-1 171 Green Inv. 1-10 (38)  0.0159 412 816 Ir-l 175 GreenInv. 1-11 (48)  0.0417 399 873 Ir-1 155 Green Inv. 1-12 (1) 0.0238 385558 Ir-2 161 Green Inv. 1-13 (6) 0.0152 407 858 Ir-3 168 Green Inv. 1-14(7) 0.0152 405 858 Ir-5 175 Green Inv. 1-15 (9) 0.0208 380 628 Ir-8 188Green Inv. 1-16 (38)  0.0159 412 816 Ir-9 185 Red Inv. 1-17 NT-1 0.05385 825 Ir-1 157 Green Inv. 1-18 NT-11  0.0435 402 831 Ir-1 150 GreenInv. 1-19 NP-1 0.037 420 725 Ir-1 153 Green Inv. 1-20 5-5 0.0323 415 821Ir-1 158 Green Inv. 1-21 1-1 0 430 553 Ir-1 152 Green Inv. 1-22 1-2 0424 475 Ir-1 155 Green Inv. Comp.: Comparative Inv.: Inventive

As is apparent from Table 2 above, inventive organic electroluminescenceelement samples comprising a light emission layer containing thecompounds in the invention emits light with high luminance. Theinventive samples have proved to be useful for an organic EL element,and the following three are pointed out.

-   (1) The organic EL element employing the host compound having an N/C    of not more than 0.05 (5%) provides high luminance. Further, when    the host compound having an N/C of not more than 0.03 (3%) is    employed, luminance is further increased.-   (2) When a host compound having a maximum fluorescence wavelength in    the wavelength regions of from 350 to 440 nm of the host compounds    having an N/C of not more than 0.05 (5%) is employed, higher    luminance is obtained.-   (3) The host compound, having an N/C of not more than 0.05 (5%), a    maximum fluorescence wavelength in the wavelength regions of from    350 to 440 nm, and a molecular weight of not less than 600, provides    the highest luminance.

The phosphorescent quantum yields of phosphorescent compounds Ir-1, 2,3, 5, 8, and 9 in 25° C. tetrahydrofuran was 0.36, 0.32, 0.27, 0.12,0.34, and 0.21. respectively

Herein, the maximum fluorescence wavelength of the fluorescent compoundas a host compound is a wavelength giving the maximum fluorescentintensity in the fluorescent spectra of a 100 nm thick layer of thefluorescent compound deposited on a glass plate.

Example 2

Organic EL element sample Nos. 2-1 through 2-21 were prepared in thesame manner as in organic EL element sample Nos. 1-1 through 1-21 inExample 1, respectively, except that α-NPD (having a maximumfluorescence wavelength of 452 nm) as a hole transporting material wasreplaced with m-MTDATXA (having a maximum fluorescence wavelength of 399nm). The resulting element samples exhibited more increased luminance.

Example 3

Organic EL element sample Nos. 3-1 through 3-21 were prepared in thesame manner as in organic EL element sample Nos. 1-1 through 1-21 inExample 1, respectively, except that the cathode was replaced with Al,and a LiF-deposited cathode buffer layer with a thickness of 0.5 nm wasprovided between the cathode and the electron transporting layer. Theluminance of emitted light from the sample was measured according toCS-1000 produced Minolta Co., Ltd. in the same manner as in Example 1.When the luminance of the organic EL element sample No. 1-8-was set at100, the relative value of the luminance of the organic EL elementsample No. 3-8 was 142. Similarly, introduction of the cathode bufferlayer was proved to be effective in the other organic EL element samplesprepared above.

Example 4

Organic EL element sample Nos. 4-1 and 4-2 were prepared in the samemanner as in organic EL element sample Nos. 1-1 and 1-8 in Example 1,respectively, except that Ir-1 was replaced with Pt-3{2,3,7,8,12,13,17,18-octaethyl-21H-Porphyrin platinum (II) (PtOEP),produced by Porphyrin Products Co., Ltd.}.

Organic EL element sample Nos. 4-3 and 4-4 were prepared in the samemanner as in organic EL element sample Nos. 1-1 and 1-7 in Example 1,respectively, except that Ir-1 was replaced with Pt-2.

The luminance of emitted light from the resulting samples was measured,and as a result, the inventive samples employing the compound in theinvention exhibited improved luminance.

A red light was emitted from the sample employing Pt-3, and a blue lightwas emitted from the sample employing Pt-2.

Example 5

Organic EL element sample No. 5-1 was prepared in the same manner as inExample 1, except that the light emission layer was replaced with alight emission layer having a thickness of 10 nm in which the respectivefive layers of a 1 nm thick layer containing fluorescent compound (7)and 1% by weight of DCM2 (light emission layer A) and a 1 nm thick layercontaining compound (7) and 10% by weight of Ir-1 (light emission layerB) were alternately layered.

A 590 nm light was emitted from DCM2 contained in the organic EL elementsample No. 5-1.

Organic EL element sample No. 5-2 was prepared in the same manner asorganic EL element sample No. 5-1 above, except that the fluorescentcompound (7) was replaced with CBP. A 590 nm light was also emitted fromDCM2 contained in the organic EL element sample No. 5-2, but itsluminance was 0.60 times that of the organic EL element sample No. 5-1employing the fluorescent compound (7). As is apparent from the above,the inventive sample provides higher luminance.

Example 6

The red light-emitting organic EL element, green light-emitting organicEL element, and blue light-emitting organic EL element prepared inExamples 1 and 4 were arranged on the same substrate, and a full colordisplay as shown in FIG. 1 was prepared which employed an active matrixsystem.

FIG. 1 is a schematic drawing of a displaying section of the full colordisplay prepared.

The displaying section A has a wiring portion including a plurality ofscanning signal lines 5 and a plurality of data signal lines 6, and aplurality of pixels 3 (a red-light emitting pixel, a green-lightemitting pixel and a blue-light emitting pixel, etc.) on the samesubstrate. The scanning signal lines 5 and the data signal lines 6 arecomposed of an electroconductive material. The line 5 and the line 6 arecrossed with each other at a right angle, and connected with the pixel 3at the crossing position (not illustrated in detail). Organic ELelements corresponding to each of the plural pixels 3 described aboveare driven by an active matrix system in which a switching transistor asan active element and a driving transistor are provided to each of theorganic EL elements. When the scanning signal is applied from thescanning signal line 5, the pixels 3 receive the image data signal fromthe data signal line 6, and emit light corresponding to the image data.A full color image can be displayed by suitably arranging the red-lightemitting pixel, green light emitting pixel and blue light emitting pixelon the same substrate.

The full color display prepared above provided a full color moving imagewith high luminance and high visibility.

Example 7

Organic-EL element sample Nos. 7-1 through 7-22 were prepared in thesame manner as comparative organic EL element sample No. 1-1 in Example1, except that the compound contained in the light emission layer andthe phosphorescent compound were replaced with those as shown in Table3.

When a direct current voltage of 9V was applied to the resulting organicEL element samples at 23° C. in an atmosphere of a dry nitrogen gas, thehalf value period. (emission lifetime) at which luminance of lightemitted from the samples was reduced to half was measured. The halflifetime (emission lifetime) of light emitted from the organic ELelement sample Nos. 7-2.through-7-22 was expressed by a relative valuewhen the half lifetime (emission lifetime) of light emitted from theorganic EL element sample No. 7-1 was set at 100. The luminance (cd/m²)was measured according to CS-1000 produced Minolta Co., Ltd. The resultsare shown in Table 3.

TABLE 3 Com- pound contained Sam- in light Phospho- Emis- ple emissionrescent sion Emission No. layer N/C compound lifetime color Remarks 7-1*TPB 0 Ir-1 100 Green Comp. 7-2 **TCPB 0.05 Ir-1  92 Green Comp. 7-3 CBP0.056 Ir-1  86 Green Comp. 7-4 TAZ 0.125 Ir-1  82 Green Comp. 7-5 OXD70.2 Ir-1  94 Green Comp. 7-6 BC 0.077 Ir-1  56 Green Comp. 7-7 TPA 0.076Ir-1  62 Green Comp. 7-8 (6) 0.0152 Ir-1 238 Green Inv. 7-9 (7) 0.0152Ir-1 225 Green Inv. 7-10 (9) 0.0208 Ir-1 204 Green Inv. 7-11 (38) 0.0159Ir-1 216 Green Inv. 7-12 (48) 0.0417 Ir-1 163 Green Inv. 7-13 (1) 0.0238Ir-2 201 Green Inv. 7-14 (6) 0.0152 Ir-3 227 Green Inv. 7-15 (9) 0.0208Ir-8 212 Green Inv. 7-16 (38) 0.0159 Ir-9 225 Red Inv. 7-17 NT-1 0.05Ir-1 101 Green Comp. 7-18 NT-11 0.0435 Ir-1 162 Green Inv. 7-19 NP-10.037 Ir-1 164 Green Inv. 7-20 5-5 0.0323 Ir-1 161 Green Inv. 7-21 1-1 0Ir-1 102 Green Comp. 7-22 1-2 0 Ir-1 108 Green Comp. Comp.: ComparativeInv.: Inventive

As is apparent from Table 3 above, inventive organic electroluminescenceelement samples comprising a light emission layer containing the hostcompound in the invention having an N/C of from more than 0 to less than0.05 (5%) provides a longer half lifetime (emission lifetime), ascompared with the comparative samples, and have proved to be useful foran organic EL element. Further, inventive organic electroluminescenceelement samples comprising a light emission layer containing the hostcompound in the invention having an N/C of from more than 0 to 0.03 (3%)provides a further longer half lifetime (emission lifetime), and haveproved to be especially useful for an organic EL element.

EFFECT OF THE INVENTION

The present invention provides an organic electroluminescent elementemitting light with high emission luminance, and a display employing theorganic electroluminescent element which emits the light at reducedpower consumption.

1. An organic electroluminescent element comprising a pair of electrodesand provided therebetween a light emission layer containing a firstfluorescent compound and a phosphorescent compound, the firstfluorescent compound having a nitrogen atom number to carbon atom numberratio in the molecule (N/C) of from more than 0 to less than 0.05,wherein the first fluorescent compound is represented by the followingformula (I), (II), or (IV):

wherein R₁ and R₂ independently represent a substituent; Ar representsan aromatic hydrocarbon ring or an aromatic heterocyclic ring, whereeach ring can have a substituent; and n is an integer from 0 to 3;

wherein R₆, R₇ and R₈ independently represent a substituent selectedfrom a halogen atom, an alkyl group, an alkoxy group, a cycloalkylgroup, an aryl group, an aryloxy group and a heterocyclic group; and n4,n5 and n6 independently represent an integer from 0 to 7; or

wherein R₁₀₁ through R₁₂₈ independently represent a hydrogen atom or asubstituent, where at least one of R₁₀₁ through R₁₀₄ represents asubstituent.
 2. The organic electroluminescent element of claim 1,wherein the first fluorescent compound has a nitrogen atom number tocarbon atom number ratio in the molecule (N/C) of from more than 0 to0.03.
 3. The organic electroluminescent element of claim 1, wherein themaximum fluorescence wavelength of the first fluorescent compound is inthe range of from 350 to 440 nm.
 4. The organic electroluminescentelement of claim 1, wherein the molecular weight of the firstfluorescent compound is not less than
 600. 5. The organicelectroluminescent element of claim 1, wherein the phosphorescentcompound has a phosphorescent quantum yield of not less than 0.01 at 25°C. in tetrahydrofuran.
 6. The organic electroluminescent element ofclaim 1, wherein a hole transporting layer containing at least onefluorescent compound or an electron transporting layer containing atleast one fluorescent compound is provided adjacent to the lightemission layer, the maximum fluorescence wavelength of the fluorescentcompound contained in the hole transporting layer or in the electrontransporting layer being in the range of from 350 to 440 nm.
 7. Theorganic electroluminescent element of claim 1, wherein the maximumfluorescence wavelength of the first fluorescent compound is in therange of from 390 to 410 nm.
 8. The organic electroluminescent elementof claim 6, wherein the maximum fluorescence wavelength of the firstfluorescent compound contained in the light emission layer and thefluorescent compound contained in the hole transporting layer or theelectron transporting layer is in the range of from 390 to 410 nm. 9.The organic electroluminescent element of claim 1, wherein one of thepair of electrodes is a cathode, and at least one cathode buffer layeris provided between the light emission layer and the cathode.
 10. Theorganic electroluminescent element of claim 1, wherein thephosphorescent compound is a heavy metal complex compound.
 11. Theorganic electroluminescent element of claim 1, wherein thephosphorescent compound is a metal complex having a metal belonging togroup VIII of the periodic table as a center metal.
 12. The organicelectroluminescent element of claim 1, wherein the phosphorescentcompound is an osmium complex, an iridium complex or a platinum complex.13. The organic electroluminescent element of claim 1, wherein theelement further comprises a fluorescent compound having a maximumfluorescence wavelength in the region longer than a maximumphosphorescence wavelength of the phosphorescent compound.
 14. A displaycomprising the organic electroluminescent element of any one of claims2,3, and 4 through
 13. 15. A full color display comprising two of theorganic electroluminescent element of any one of claims 2, 3, and 4through 13, wherein the two elements are arranged on the same substrate,and emit light having a maximum emission wavelength different from eachother.
 16. The organic electroluminescent element of claim 1, whereinthe substituent of R₁ and R₂ in Formula (I) independently is selectedfrom a halogen atom, an alkyl group and an alkoxy group; and thesubstituent of R₁₀₁ through R₁₂₈ in Formula (IV) independently isselected from an alkyl group, an alkoxy group, a cycloalkyl group, anaralkyl group, an aryl group, an aryloxy group and an arylamino group.